1. Field of the Invention
The present invention relates to an image forming device that forms a color image by superimposing toner images of plural colors, and more particularly to an image forming device wherein a photoreceptor drum having an electrostatic latent image formed thereon and a developing device for making the latent image visible are provided for every color, whereby each of the formed toner images is laid over a recording medium transported on an intermediate transfer belt or a transport belt to thereby be transferred thereon.
2. Description of the Related Art
There have been proposed some types of image forming device for forming a color image by use of a toner. These include the followings of:
(a) the type having a single photoreceptor drum, wherein a toner image of a different color is successively formed thereon so as to be superimposed and the formed images are transferred all together onto a recording medium;
(b) the type having a single photoreceptor drum, wherein a toner image of a different color is successively formed thereon, the formed toner image of each color is transferred onto an intermediate transfer medium or on a recording medium, and then, the toner images are superimposed onto the intermediate transfer medium or on the recording medium;(c) the so-called tandem type having photoreceptor drum and developing device for every color, wherein toner images are superimposed to be transferred onto a recording medium transported by an intermediate transfer belt or a transport belt.
Among these types, the tandem type image forming device is fast in image forming speed, so that it becomes a mainstream of a color image forming device. Such a tandem type image forming device has a photoreceptor drum, a developing device, a charging device, a cleaning device and a transferring device for every color of yellow, magenta, cyan and black. Each of these used image forming units generally has the specification totally same as one another. The use of the same image forming units allows to facilitate the management upon the manufacture and the supply of the components, thereby being capable of fabricating the device with low cost. Further, a uniform image can be formed for each color, and further, the maintenance management, setting and adjustment can be facilitated.
However, the images formed by use of the image forming device described above include a color image and a monochrome image of only a character or the like, so that the device is usually used to switch these modes. When a monochrome image is formed with the image forming device described above, only a unit for forming a black toner image is driven to form an image. Therefore, in the case where the photoreceptor drum is used as an image-bearing member, the used amount of the photoreceptor drum for black is remarkably more increased than that of the photoreceptor drum for forming the toner image of another color, with the result that only the photoreceptor drum for black is required to be exchanged at an early stage. In view of the abovementioned circumstance, an image forming device has been proposed wherein a large-diameter drum is used only for a photoreceptor drum for black (Patent Reference 1). In this device, the useful period of the photoreceptor drum for black is prolonged since the large-diameter photoreceptor drum allows to decrease the number of rotation to thereby reduce the number of times for forming an image on the peripheral surface. Therefore, the useful period of the photoreceptor drum for black becomes close to the useful period of the photoreceptor drums for another colors, thereby facilitating the management of the device.
It is to be noted that the image forming unit for forming the black toner image is frequently disposed at the most downstream side in the moving direction of the intermediate transfer belt or the transport belt in such tandem type image forming device. This is because this configuration shortens the period from the start of the formation of the toner image to the secondary transfer onto the recording medium via the transfer of the formed image onto the intermediate transfer belt, thereby improving the speed of the image formation.
[Patent Reference 1]
Japanese Published Unexamined Patent Application No. 2000-242057
There are the following subjects that are desired to be solved in the image forming device as mentioned above having photoreceptor drum, developing device and transferring device for every color of toner, wherein the diameter of each photoreceptor drum is different with respect to the color of toner.
When the toner image formed on the photoreceptor drum is transferred to the recording sheet carried on the intermediate transfer belt or the transport belt, its condition is different depending upon the difference in the diameter of the photoreceptor drum. The difference in the transfer condition is remarkable in a transferring device wherein a conductive roller is brought into contact with a photoreceptor drum from the back surface of the intermediate transfer belt or the transport belt to push the photoreceptor drum and a bias voltage for the transfer is applied between the conductive roller and the photoreceptor drum. Specifically, the intermediate transfer belt on which the toner image is transferred gets close to the peripheral surface of the photoreceptor drum, and it is separated from the peripheral surface after it comes in contact therewith. The regions where the intermediate transfer belt gets close to the peripheral surface of the photoreceptor drum and is separated therefrom and the speed for passing through these regions are different. When the bias voltage for the transfer is applied, a discharge occurs at a minute space immediately before the contact and immediately after the separation. The state of this discharge is varied when the diameter of the photoreceptor drum is different, so that the transferability becomes different. Further, the change in the diameter of the photoreceptor drum allows to cause a difference in contact area between the photoreceptor drum and the intermediate transfer belt or the like, with the result that, when the transfer roller is pushed to the photoreceptor drum with the same force, the contact pressure is changed.
As described above, the transferring condition is made different in electrical factors and pressure factors by the difference in the diameter of plural photoreceptor drums used in the same device, so that the transferability is changed for every color of each toner. The difference in the transferability for every color of each toner as described above causes to change a tone when the toner images are superposed, thereby deteriorating an image quality.